Page 61 - Synthetic Fuels Handbook

NATURAL GAS 49

The need to refrigerate the solvent is the main disadvantage of the process, resulting in high
capital and operating costs.
The Sulfinol process, developed in the early 1960s, is a combination process that uses
a mixture of amines and a physical solvent. The solvent consists of an aqueous amine and
sulfolane. In operation, this process is in many respects identical to the familiar amine
method and its equipment components similar to those found in amine units. The main dif-
ference is that while the conventional amine process employs a fairly diluted concentration
of amine in water, removing the acid gas by chemical reaction, the Sulfinol system uses a
mixture of highly concentrated amine and a physical solvent removing the acid gases by
physical and chemical reactions. The concentrations of the amine and the physical solvent
vary with the type of feed gas in each application. Common Sulfinol mixtures are in the
range of 40 percent amine (also called DIPA), 40 percent sulfolane (an organic solvent),
and 20 percent water.

2.7.3 Metal Oxide Processes
Hydrogen sulfide scavengers have been used for many years. An example is the iron sponge
process (dry box process) that is the oldest and still the most widely used batch process for
sweetening of natural gas and natural gas liquids (Duckworth and Geddes, 1965; Anerousis
and Whitman, 1984; and Zapffe, 1963). The process was implemented during the nine-
teenth century and has been in use in Europe and the United States for over 100 years.
Hydrogen sulfide scavengers are appropriate for use at the low concentrations of hydrogen
sulfide where conventional chemical absorption and physical solvents are not economical.
During recent years, hydrogen sulfide scavenger technology has been expanded with many
new materials coming on the market and others being discontinued. Overall, the simplicity
of the process, low capital costs, and relatively low chemical (iron oxide) cost continue to
make the process an ideal solution for hydrogen sulfide removal.
The sponge consists of wood shavings impregnated with a hydrated form of iron
oxide. The wood shavings serve as a carrier for the active iron oxide powder. Hydrogen
sulfide is removed by reacting with iron oxide to form ferric sulfide. The process is usu-
ally best applied to gases containing low to medium concentrations [300 ppm (parts per
million)] of hydrogen sulfide or mercaptans. This process tends to be highly selective and does
not normally remove significant quantities of carbon dioxide. As a result, the hydrogen
sulfide stream from the process is usually high purity. The use of iron sponge process for
sweetening sour gas is based on adsorption of the acid gases on the surface of the solid
sweetening agent followed by chemical reaction of ferric oxide (Fe O ) with hydrogen
3
2
sulfide:
2Fe O + 6H S → 2Fe S + 6H O
3
2
2
2 3
2
The reaction requires the presence of slightly alkaline water and a temperature below
43°C (110°F) and bed alkalinity should be checked regularly, usually on a daily basis. A
pH level on the order of 8 to 10 should be maintained through the injection of caustic soda
with the water. If the gas does not contain sufficient water vapor, water may need to be
injected into the inlet gas stream.
The ferric sulfide produced by the reaction of hydrogen sulfide with ferric oxide can be
oxidized with air to produce sulfur and regenerate the ferric oxide:
2Fe S + 3O → 2Fe O + 6S
2
3
2
2 3
S + 2O → 2SO 2
2
2